Dynamic size computation and caching for cells.

RZCellSizeManager is an object used to streamline the calculation, caching, and retrieval of sizes for UICollectionView cells and heights for UITableView cells. It pairs particularly nicely with AutoLayout but can be used anytime you want to simplify management of cell sizes.

Add the pod to your Podfile:

pod 'RZCellSizeManager', '~>1.1'

To get the CoreData or RZCollectionList extensions, use (respectively):

pod 'RZCellSizeManager/CoreDataExtensions', '~>1.1'
pod 'RZCellSizeManager/RZCollectionListExtensions', '~>1.1'

Copy the RZCellSizeManager folder into your project. The only files you need are RZCellSizeManager.h and RZCellSizeManager.m

To use RZCellSizeManager you first must create an instance of it.

@property (strong, nonatomic) RZCellSizeManager *sizeManager;

self.sizeManager = [[RZCellSizeManager alloc] init];

Once we have an instance we can then register different cell classes/nibs.

Depending on what your use case is there are a few different methods to do this (See the in class documentation for more methods):

- (void)registerCellClassName:(NSString *)cellClass
                 withNibNamed:(NSString *)nibNameOrNil
           forReuseIdentifier:(NSString *)reuseIdentifier
       withConfigurationBlock:(RZCellSizeManagerConfigBlock)configurationBlock; 
       
- (void)registerCellClassName:(NSString *)cellClass
                 withNibNamed:(NSString *)nibNameOrNil
               forObjectClass:(Class)objectClass
       withConfigurationBlock:(RZCellSizeManagerConfigBlock)configurationBlock;
       

The first method above will work similarly to how your UITableView interaction will work. You give RZCellSizeManager a cell class name, an optional nib name, and a reuseIdentifier. The last parameter is a configuration block which you will use to configure your cell, given an optional model object and an indexPath.

[self.sizeManager registerCellClassName:NSStringFromClass([TableViewCell class]) 
                           withNibNamed:nil 
                     forReuseIdentifier:[TableViewCell reuseIdentifier] 
                     configurationBlock:^(TableViewCell* cell, id object) {
                         [cell setCellData:object];
                     }];

If you pass in a reuseIdentifier of nil it will work fine so long as you only have one cell type.

The second approach is to register with an object class. Based on a paticular object class passed in when asking for the cell size, the manager will choose the correct cell for you.

[self.sizeManager registerCellClassName:NSStringFromClass([TableViewCell class]) 
                           withNibNamed:nil 
                         forObjectClass:[CellData class] 
                     configurationBlock:^(TableViewCell *cell, CellData *object) {
                         [cell setCellData:object];
                     }];

Mixing these two different methods will result in unsupported behavior.

In this case we are setting an object on the cell which will set two different labels. Both of these labels are configured with autolayout so they will adjust their size depending on the content. Here is an example of the setCellData: method

// Using AutoLayout
- (void)setCellData:(CellData *)cellData
{
    self.titleLabel.text = cellData.title;
    self.descriptionLabel.text = cellData.subTitle;
}

Then just implement the UITableViewDelegate Method and ask the sizeManager for the size or height at a particular index path.

For the ReuseIdentifier approach we just pass in an additional parameter as the ReuseIdentifier for the cell that we want to use to get our height based off the indexPath.

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
   // Retrieve our object to give to our size manager.
   id object = [self.dataArray objectAtIndex:indexPath.row];
   return [self.sizeManager cellHeightForObject:object indexPath:indexPath cellReuseIdentifier:[TableViewCell reuseIdentifier]];	
}

For the object class approach, RZCellSizeManager will figure out the correct cell to use based on the class of the provided object, so this call is even simpiler.

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
   // Retrieve our object to give to our size manager.
   id object = [self.dataArray objectAtIndex:indexPath.row];
   return [self.sizeManager cellHeightForObject:object indexPath:indexPath];
}

And you're done! All of your cell's sizes will be cached so that any future calls will be lightning-fast.

If you are using RZCellSizeManager and you data changes you will need to invalidate its cache so that it can compute the new heights. This can be done by giving it a specific index path, or you can just invalidate the entire cache.

- (void)invalidateCellHeightCache;
- (void)invalidateCellHeightAtIndexPath:(NSIndexPath *)indexPath;
- (void)invalidateCellHeightsAtIndexPaths:(NSArray *)indexPaths;

Check out the demo project for a simple example of how to use the RZCellSizeManager and feel free to add issues and pull requests if you have good ideas for future enhancements. sizeManager Also see this link for a blog post about RZCellSizeManager.